In bacteria and yeast, many of the genes important in the cellular response to DNA damage are induced by such damage. Based on observations in bacteria and yeast, we predicted that, if similar responses occur in mammalian cells, such genes will have these characteristics: 1) low abundance transcripts; 2) rapid induction; 3) induction of only 2-10 fold; 4) induction specific for DNA damaging agents. In order to isolate cDNA clones for DNA-damage- inducible (DDI) transcripts which may be induced only several fold, we used hybridization subtraction at low ratios of RNA:cDNA. With this approach, 49 different cDNA clones were isolated which coded for low abundance transcripts that were rapidly induced by the DNA- damaging agent, UV radiation, in Chinese hamster cells. Most were only induced by DNA damage and not other types of stress such as heat shock. The cDNA clones were divided into 2 general classes; in the first, induction occurred only after treatment of cells with UV or UV-mimetic agents, while in the second class, induction was seen with many agents which produce high levels of DNA base damage such as alkylating agents. By DNA sequence analysis most of our cDNA clones did not correspond to any known genes. Our cDNA clone, DDIA18 had 2 localized areas of homology with a known DNA single strand binding protein. Another of our cDNA clones, DDIA153, was found to code for RNA which was increased either by DNA damage or inhibition of cell growth. DDIA153 may code for a growth cessation gene which could have a protective effect on cells exposed to DNA- damaging agents. In collaboration with I. Hickson and L. Barrows, expression of our DDI transcripts in DNA repair mutants has been investigated. Several examples of both increased and decreased expression transcription in the mutant cells has been observed. Increased expression of certain DDI genes have been found in C- 14Cos mouse liver in collaboration with D. Nebert; this mutant contains a deletion in chromosome 7 which may regulate one or more stress responses. The functions of most of our DDI sequences are unknown, but it is probable that the protein products of at least some of these transcripts play a role in the cellular response to DNA damage.